The present invention relates to a fuel injection system for a diesel engine, and more particularly to a rotary valve device for distributing fuel to intensifying unit injectors.
U.S. Pat. No. 4,440,134 granted to Kiyoharu Nakao discloses a known conventional fuel injection system employing a rotary valve device. According to FIG. 1 given in the specification of the same U.S. patent, a high-pressure fuel supply pump 10 is driven by the internal combustion engine 12 to supply fuel from a fuel tank 14 at a high pressure. The pressure of the high-pressure fuel discharged by the high-pressure pump 10 changes pulsatively over a wide range. Accordingly, the fuel discharged by the high-pressure pump 10 is accumulated temporarily in an accumulator 16, and then supplied to the rotary valve 18 after reducing the pressure variation. The rotary valve 18 is driven in synchronism with the cam shaft of the engine 12 to distribute the high-pressure fuel to the injectors 20.sub.1, 20.sub.2 . . . and 20.sub.6. As shown in FIG. 3 of the same U.S. Patent specification, the rotary valve 18 has a cylindrical rotary sleeve 24 fitted for rotary motion through a predetermined angle in a housing 22 HS and a rotary shaft 26 fitted rotatably and concentrically in the rotary sleeve RL. Ports P.sub.1, P.sub.2 and P.sub.3 and ports Q.sub.1, Q.sub.2 and Q.sub.3 are formed correspondingly at predetermined positions in the housing 22 and the rotary sleeve RL 24 respectively. Circumferential recesses 28 and 30 are formed at predetermined positions on the circumference of the rotary shaft 26 to change over between an injecting range and a metering range (a range to decide the amount of fuel to be injected). The rotary shaft 26 is driven for rotation in synchronism with the cam shaft, not shown, of the engine 12, for example, in a clockwise direction.
In the conventional rotary valve device, only a single metering groove (a groove to decide the amount of fuel to be injected) is formed in the rotary shaft which also is designated as a rotary spool. Therefore, four to six unit injectors interfere with each other in metering the fuel, and hence the disturbance occurred in one of the unit injectors affects the rest of the unit injectors to disturb the stability of fuel metering.
The fuel metering process in the prior art intensifying unit injectors will be described hereunder in connection with FIG. 1.
At the completion of fuel injection by a unit injector 10a, the piston 12 and the plunger 13 of the unit injector 10a are moved to and positioned at the respective bottom positions and a fuel supply pipe 15a and the upper pressure chamber 14 of the unit injector 10a are filled with the high-pressure fuel.
A fuel supply port 25 is formed at a position on the circumference of the rotary spool 22 of a rotary valve 20. One end of the fuel supply port 25 communicates with a fuel passage 24 which communicates with a fuel supply pipe 27. The other end of the fuel supply port 25 faces a conduit 15d at the time of fuel injection from a unit injector 10d to depress the piston 12 and the plunger 13 against the resilient force of the metering spring 11. A metering groove 30 is formed at another position on the circumference of the rotary spool 22 to connect the upper pressure chamber 14 of the unit injector 10a through a conduit 15a to a drain pipe 28 after the completion of fuel injection from the unit injector 10a.
When the conduit 15a communicates with the drain pipe 28 by means of the metering groove 30 of the rotary spool 22 after the completion of the fuel injection, the fuel remaining in the upper pressure chamber 14 of the unit injector 10a is pushed out by the resilient force of the metering spring 11 and is drained through a metering valve 29 provided in the drain pipe 28. The amount of fuel to be injected is dependent on the upward return travel of the plunger 13. Accordingly, if the opening of the metering valve 29 is small, the return travel of the piston 12 and the plunger 13 is limited to a small extent by an increased flow resistance, and thereby the amount of fuel injection is reduced. On the contrary, if the opening of the metering valve is large, the flow resistance decreases, and thereby the piston 12 and the plunger 13 are allowed to make an increased return travel, and hence the amount of fuel injection is increased.
Basically, the amount of fuel injection is controlled according to the above-mentioned conception. In a practical internal combustion engine, the amount of fuel injection is controlled by regulating the opening of the metering valve 29 through the feedback of the output of the engine. Accordingly, the relation between the amount of fuel injection and the opening of the metering valve needs to be monotonic.
Nevertheless, if the opening of the metering valve is fixed, generally the amount of fuel injection decreases when the engine speed increases, which causes the engine speed to decrease again, whereas if the engine speed decreases the amount of fuel injection increases, which causes the engine speed to increase again. That is, the relationship between the amount of fuel injection and the engine speed becomes a negative slope so that the engine speed is stabilized. However, within a certain range of engine speed, such a relationship becomes a positive slope, and hence the engine speed becomes unstable. Accordingly, the control of the amount of fuel injection is generally complicated.
The reason why the relationship between the amount of fuel injection and the engine speed becomes a positive slope is due to a fact that the amount of fuel drain through the metering valve 29 does not varies monotorously with respect to the engine speed, because there is unevenness between respective amounts of fuel reserved within the upper pressure chambers 14 of unit injections 10a . . . and 10f and within fuel supply pipes 15a . . . and 15f owing to the unsteady state of the fuel within the upper pressure chambers 14 of the unit injectors 10a . . . and 10f and within the fuel supply pipes 15a . . . and 15f.